Flow meter with ultrasound transducer directly connected to and fixed to measurement circuit board

ABSTRACT

An ultrasound flow meter unit arranged to measure a fluid flow rate with one or more ultrasound transducers (606), and a circuit board (602) with an electronic circuit arranged to operate the ultrasound transducer(s) (106, 306). The ultrasound transducer (606), e.g. in the form of a piezo-electric element, is mechanically fixed to the circuit board (602) by a first electrically conducting fixing means which additionally serve(s) to provide an electrical connection between an electrical terminal of the transducer (606) and the electronic circuit. Hereby a functional flow measurement unit (600) is provided which can be tested prior to assembly with a flow meter housing etc. Preferably, a set of ultrasound transducers (106, 306) are soldered directly onto electrically conducting paths (631) on a surface of the circuit board (602) being in electrical connection with the electronic circuit, e.g. in a single SMT mounting process together with mounting of all other electronic components on the circuit board (602). Through-going openings (630, 640) of the circuit board (602) in the vicinity of these conducting paths (631) serve to provide thermal elasticity. A metal clip soldered to the circuit board (602) may serve to electrically connect a second electrical terminal of the transducer (606) to the electronic circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority to and is a U.S.National Phase Application of PCT International Application NumberPCT/DK2010/050068, filed on Mar. 25, 2010, designating the United Statesof America and published in the English language, which is anInternational Application of and claims the benefit of priority toEuropean Patent Application No. 09157174.5, filed on Apr. 2, 2009, andEuropean Patent Application No. 09179932.0, filed on Dec. 18, 2009. Thedisclosures of the above-referenced applications are hereby expresslyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The invention relates to an ultrasound flow meter for ultrasonicmeasurements of fluid flow, such as ultrasound flow meters forconsumption meters. In particular the invention provides an ultrasoundflow meter with measurement circuit board with an ultrasound transducerelectrically connected and fixed thereto.

BACKGROUND OF THE INVENTION

Normally, ultrasound flow meters suited for measuring a fluid flow inconnection with charging of a consumed quantity (e.g. heat, cooling,water or gas) will have a housing, which can be metallic or polymeric,with a cavity in the form of a through-going hole for receiving a fluidflow to be measured. Connection means to other fluid flow elements arepresent in each of the housing ends.

In the housing a number of ultrasound transducers are installed formeasuring the velocity of the fluid flow. In most flow meters twoultrasound transducers are used for sending, respectively receiving, anultrasound signal, but versions with one ultrasound transducer, as wellas versions with more than two ultrasound transducers, are seen.Furthermore, a measurement circuit, i.e. an electronic circuit foroperation of the ultrasound transducers, is typically mounted in aseparate enclosure and fastened to the housing. Most often theelectronic circuit is implemented on a Printed Circuit Board (PCB).

The ultrasound transducers can be electrically connected to theelectronic control circuit in various ways. Common methods beingsoldering ends of connection wires onto the ultrasound transducersdirectly and onto the PCB respectively, or using a plug connectionbetween the same. However, one common problem with prior art ultrasoundflow meters is that, during manufacturing, it is required that the flowmeter is completely assembled before a meaningful test of the flow metercan be performed. Species that fail in such test procedure are thusrevealed at a late state in the manufacturing process, and thus assemblytime is wasted for such species.

SUMMARY OF THE INVENTION

In view of the above, it may be seen as an object of the presentinvention to provide an improved ultrasound flow meter for ultrasonicmeasurement of a fluid flow and an improved method of manufacturing of aflow meter which enables testing prior to final assembly.

Accordingly, the invention provides, in a first aspect, an ultrasoundflow meter unit according to claim 1.

Thus, an improved ultrasound flow meter unit for ultrasonic measurementof a fluid flow is provided. In particular, it may be seen as animprovement that integration of parts involved with the positioning andthe mounting of the ultrasound transducer is achieved, as the circuitboard, together with the fixing means, serves to both fix andelectrically connect the ultrasound transducers to the circuit board,thus forming a mechanically separate unit which can be easily testedprior to the final assembly steps of mounting this unit on a flow meterhousing, such as a housing with a measuring tube arranged inside athrough-going opening, or such as a housing with the measuring tubeintegrated therein. This is essential for effective production, sincethe mechanically separate unit including transducer(s) and circuit boardcan be tested before final assembly of the mechanically separate unitand the housing. Hereby, the production cost can be lowered sincefailing species are revealed at an early state, and in addition therather complex handling of wires and connecting of wires betweentransducer(s) and circuit board in automated manufacturing has beeneliminated. Also, a more reliable flow meter can be achieved due tofewer parts and connections. Moreover, this can also lead to an increasein lifetime of the ultrasound flow meter. In addition, higher precisionis possible as a result of reduced tolerances.

Furthermore, a direct mounting of the ultrasound transducer to thecircuit board reduces the number of components required for fixing andelectrically connecting the ultrasound transducer to the circuit board,and thus eliminating components will help to save space and therebyallow for a compact flow meter. Especially, it is preferred that theultrasound transducer in the form of a monolithic body of piezo-electricmaterial is fixed and electrically connected directly on a surface ofthe circuit board, thus resulting in a very compact ultrasoundmeasurement unit with few single components.

In addition, by using fixing means serving to both mechanically fixingand electrically connecting the ultrasound transducer enableselimination of several steps in the manufacturing process involvingmounting of the ultrasound transducers. Furthermore, by a direct andpermanent fixing and electrical connection of at least one electricalterminal of the ultrasound transducer to the measurement circuiteliminates the need for handling of fragile wire connection of theultrasound transducer. Hereby the manufacturing process is facilitated,and a source of functional error in the final flow meter is eliminatedsince wire connections are mechanically vulnerable and can easily break,thus causing break-down of the flow meter.

Still further, by the defined fixing and electrical connection ofultrasound transducer(s), it is possible to provide a compact ultrasoundmeasurement unit which includes an electrical interface such as in theform of a one-plug connection comprising two, three or four electricalpins serving to provide all necessary external electrical connections,preferably including one electric terminal communicating a measuredfluid flow rate, such as in the form of electric pulses with a ratecorresponding to the measured fluid flow rate. Hereby, it is possible toprovide a compact ultrasound measurement unit is provided which is botheasy to test, but which also provides a high degree of versatility forintegration into a large variety of applications. Such flow meter unitmay include a battery electrically connected to the circuit board so asto allow the flow meter unit to operate as a stand-alone measurementunit. However, for other applications, it may be preferred toelectrically power the electronic circuits on the circuit board throughthe electrical interface terminals. In some applications, the electricalinterface of the unit may be in the form of a wireless communicationmodule, e.g. a wireless Radio Frequency module for transmitting datarepresenting the measured fluid flow rate.

When used as part of a consumption meter, the circuit board of the flowmeter may include all of or at least part of the necessary electroniccomponents required to implement a calculation circuit arranged todetermine a consumed amount of a physical entity, based on the fluidflow rate from the flow meter. However, it may also be preferred toprovide the calculation circuit facilities in a separate unit connectedto the flow meter with one of the mentioned electrical interfaceoptions.

Preferably, the fixing means includes solder or electrically conductiveglue or adhesives, and especially, the ultrasound transducer(s) can befixed and electrically connected in a Surface Mounting Technology (SMT)process, e.g. together with the mounting of electronic componentsforming the electronic circuits on the circuit board. Such process canbe performed manually, however using SMT mounting, the flow meter issuited for automated mass production using known production equipmentwith high precision positioning of the ultrasound transducers. Thus, intwo-transducer versions, the ultrasound transducer can be positionedprecisely relative to each other and relative to the circuit board,thereby facilitating final assembly with a flow meter housing preparedfor receiving the circuit board and the transducers.

The first fixing means preferably comprises at least one of: solder,electrically conducting glue, and an electrically conducting clip. E.g.different types of these fixing means may be used to fix and connect thefirst and a second electrical terminal of the ultrasound transducer.Preferably, the piezo-electric element of the ultrasound transducer isdirectly fixed and electrically connected to a conductive path on asurface of the circuit board, thereby providing a very compact andsimple combined mechanical and electrical interconnection of the circuitboard and the ultrasound transducer.

Preferably, the first fixing means serves to electrically connect thefirst electrical terminal of the first ultrasound transducer to anelectrically conducting path on a surface of the circuit board, whereinthe electrically conducting path is electrically connected to theelectronic circuit. Especially, said conducting path may be shaped tofit at least part of a geometry of the first electrical terminal of thefirst ultrasound transducer.

The first ultrasound transducer may have a substantially flat surfacearranged for facing the circuit board, when fixed thereto. Especially,the first ultrasound transducer may have a substantially circular orrectangular outer shape and have first and second electrical terminalsarranged on respective opposite flat surfaces. The circular shapedultrasound transducer may be preferred, since it is a very easy shape tohandle for automated manufacturing machinery, such as robots. As analternative, the first ultrasound transducer may have a circular shapeand may have the first and second electrical terminals placedconcentrically in relation to each other. This enables full electricalconnection of the ultrasound transducer to the circuit board in a simpleSMT mounting process using only one side of the transducer for bothelectrical terminals.

In some embodiments, a first conducting path is electrically connectedto the first electrical terminal of the first transducer, and a secondconducting path on the surface of the circuit board serves toelectrically connect the electronic circuit with a second electricalterminal of the first ultrasound transducer. Especially, a metal clipmay serve to electrically connect the second conducting path to thesecond electrical terminal of the first ultrasound transducer, e.g.while the first electrical terminal of the transducer is directlysoldered or conductively glued to the first conducting path. Especially,such metal clip may be soldered to the second conducting path, e.g. inone and the same SMT process as all other components on the circuitboard, and wherein the metal clip is shaped such that electricalconnection to the second electrical terminal of the first ultrasoundtransducer requires application of an external force. Thus, the metalclip may be bend into shape such that it is not in contact with theelectrical terminal of the ultrasound transducer during mounting. Herebythe clip is suited for SMT mounting, and upon mounting on a housing, thetransducer(s) will face a surface of the housing, and thus the clip willbe pressed against the second electrical transducer terminal and thusprovide contact.

Alternative to the metal clip solution, both of the first and secondconducting paths may be electrically connected to the respective firstand second electrical terminals by means of solder or electricallyconducting glue.

The circuit board may have one or more through-going openings partly orentirely covered by the first ultrasound transducer when fixed to thecircuit board, such as a part of the conducting path being arranged invicinity of at least one of said through-going opening(s) in the circuitboard. Hereby it is possible for the circuit board to adapt to thermalextensions and contractions of the piezo-electric ultrasound transducerelement.

In many applications, such as small flow meters, the meter unit mayfurther comprise a second ultrasound transducer, wherein the electroniccircuit is further arranged for operating the second ultrasoundtransducer, wherein the second ultrasound transducer is mechanicallyfixed to the circuit board by a second electrically conducting fixingmeans which additionally serve(s) to provide an electrical connectionbetween at least one electrical terminal of the second ultrasoundtransducer and the electronic circuit, and wherein both of the first andsecond ultrasound transducers are mechanically fixed to the same side ofthe circuit board. A reflector may be arranged so as to guide ultrasoundsignals between the two ultrasound transducers in a direction up-streamor down-stream of the fluid flow.

In such two-transducer versions, a substantially watertight membrane orcasing may be arranged around the circuit board and the first and secondultrasound transducers so as to provide a highly versatile stand-aloneflow meter unit. Due of the water-tight casing, the critical electricalparts are sealed against humidity which is a problem, e.g. in coolingconsumption meters. A plug with a plurality of electrical terminals maypenetrate through said membrane so as to allow external connection tothe electronic circuit. At least a part of said membrane may beelectrically conducting and serve to electrically connect a secondelectrical terminal of the first ultrasound transducer with a secondelectrical terminal of the second ultrasound transducer. Hereby, themembrane can serve to electrically connect the second electricalterminals of the two transducers to the electronic circuit, e.g. to forma common electrical ground connector for the transducers. Thus, inembodiments where the first electrical terminals of both transducers aresoldered to the circuit board, the need for wires or clips iseliminated.

The circuit board is preferably substantially plane at least in an areawhere the first ultrasound transducer is fixed to the circuit board,such as the circuit board being substantially plane in its entireextension. A normal one or multi-layer printed circuit board (PCB) maybe used. Preferably, the circuit board is plane and stiff enough toensure that two ultrasound transducers mounted thereon are fixed withinan acceptable tolerance with respect to relative angular displacement.In other words, a direction of ultrasound emitted from the twoultrasound transducers is preferably not influenced significantly abending of the circuit board, and thus both ultrasound transducer shouldbe able to emit ultrasound signals perpendicularly to the surface of theplane circuit board.

In two-transducer embodiments, both of the first and second ultrasoundtransducers may be arranged to emit ultrasonic signals in a directionsubstantially perpendicular to a direction of fluid in the measurementtube of the ultrasound flow meter. However, in some applications, e.g.in fluid flow meters suited for large fluid flows, it may be preferredto arrange the transducer such that it emits ultrasound signals inanother angle, e.g. 20°-80°, with the fluid flow direction.

The ultrasound flow meter might have its housing formed by metal, suchas brass or stainless steel. In another embodiment, the ultrasound flowmeter has its housing formed by a polymeric material. An advantage of apolymeric material is that it is possible to avoid openings in thehousing for the ultrasound transducers, since an ultrasonic signal canbe transmitted through a polymer material with the right physical anddimensional properties. Sealing can thus be eliminated, resulting inimproved imperviousness of the flow meter, and the pressure forces fromthe fluid flow will not load the ultrasound transducers but the flowmeter housing only.

The invention provides a highly versatile stand-alone unit for a largevariety of applications where a flow rate of a fluid is to be measured.A simple electrical interface can be provided, e.g. where the measuredflow rate is output as pulses. This interface may be reduced down to onesingle electrical pin, see e.g. European patent application publicationNo. EP 2 042 837, e.g. with part of the casing being a metal serving aselectrical ground connector. Hereby, the unit is easy to fit intodifferent applications. In some embodiments, a battery is providedwithin the water-tight casing, thus allowing a completely self-containedflow meter unit with a simple electric interface.

Further, the range of applications is extended due to the rather compactsize of the unit, especially in versions where the electronic circuit isrefined to allow a high measurement accuracy with a limited distancebetween the first and second ultrasound transducers, e.g. as describedin European patent application EP 2 339 301. This means that the unitcan be used in applications where only a limited space is available forthe flow rate sensor.

Examples of relevant applications are: consumption meters for chargingpurposes, measurement of blood flow in medical examination orsurveillance, ingredient flow within the food industry, flow of water,air or fuel in a machinery (e.g. a vehicle), flow provided to singlesources or groups of heating/cooling elements (or the relatedheating/cooling power based on measurement of the flow) as feedback toheating/cooling control systems.

In a second aspect, the invention provides flow meter arranged tomeasure a flow rate of a fluid flowing through a measuring tube, whereinthe flow meter comprises

-   -   a housing with the measuring tube arranged inside, and    -   a ultrasound flow meter unit according to the first aspect        arranged in relation to the housing.

The flow meter may be in the form of a consumption meter being: aheating meter, a cooling meter, a water meter, or a gas meter. The flowmeter may be more or less integrated with parts implementing theconsumption meter function, e.g. housed within one common casing orwired or wirelessly connected in separate casings.

In a third aspect, the invention provides a method of manufacturing anultrasound flow meter unit of the first aspect.

Preferably, the first electrically conducting fixing means is solder,and wherein the method comprises soldering a metal clip to anelectrically conducting path on a surface of the circuit board being inelectrical connection with the electronic circuit, wherein the metalclip is arranged for providing electrical connection to a secondelectrical terminal of the first ultrasound transducer.

Preferably, step c) and a step of mounting electronic components of theelectronic circuit to electrically conducting paths on a surface of thecircuit board are performed in one SMT process. Especially, said SMTprocess may comprise mechanically fixing a first ultrasound transducerto the circuit board by a second electrically conducting fixing meanswhich additionally serve(s) to provide an electrical connection betweena first electrical terminal of the second ultrasound transducer and theelectronic circuit, or to an electrically conducting path on a surfaceof the circuit board being in electrical connection with the electroniccircuit.

In a fourth aspect, the invention provides a method of manufacturing aflow meter of the second aspect.

An advantage of this assembly procedure is that it can be very easilyperformed and that it can be automated. As only few steps and handlingof only few components are required, the process can be carried out inshort time.

A specific method of manufacturing comprises mounting the ultrasoundtransducer to the circuit board by means of one or more clips, such asmetal clips. This assembly procedure is simple yet with good positioningaccuracy. Few different components are used, and few assembly proceduresare involved.

A preferred method of manufacturing comprises soldering the ultrasoundtransducer to the circuit board, such as by means of an SMT solderingprocess. By SMT positioning of the ultrasound transducer the highaccuracy of the SMT machinery is achieved. The accuracy of positioningis in the order of fractions of a millimeter, such as below 1.0 mm, suchas below 0.1 mm, such as below 0.01 mm. Another advantage is that theelectrical connection between the circuit board and the at least oneultrasound transducer can be established in connection with mounting,thus eliminating two separate processes for fixing and electricallyconnecting, respectively.

Another preferred method of manufacturing comprises a step of testing afunction of the mechanically separate flow meter unit prior to mountingthis unit to the housing arranged for fluid flow. This method ofmanufacturing is advantageous since testing the function of themechanically separate unit before mounting the mechanically separateunit to a housing arranged for fluid flow can reveal malfunction. Incase of malfunction the product can be scrapped before the cost of theassembly procedure is added and this potentially lowers the overallproduction costs.

It is appreciated that advantages described for the first aspect appliesas well for the other mentioned aspects. Further, embodiments of thefirst aspects may in any way be combined with the other mentionedaspects.

These and other aspects, features and/or advantages of the inventionwill be apparent from and elucidated with reference to the embodimentsdescribed hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIGS. 1a and 1b illustrate a mechanically separate unit comprising acircuit board and two ultrasound transducers mounted outside a plane ofthe circuit board in an assembled and in a pre-assembly state,respectively,

FIGS. 2a and 2b illustrate a circuit board and ultrasound transducersbefore and after assembly into a mechanically separate part consistingof circuit board and ultrasound transducers mounted in an SMT process,

FIG. 3 illustrates an ultrasound flow meter with a flow meter housingmade of metal and a mechanically separate unit comprising a circuitboard and two ultrasound transducers,

FIG. 4 illustrates an ultrasound flow meter with a flow meter housingmade of polymer and a mechanically separate part comprising a circuitboard and two ultrasound transducers, and

FIGS. 5a and 5b illustrate one SMT embodiment with an annular conductingpath arranged around a central hole in the circuit board,

FIGS. 6a and 6b illustrate another SMT embodiment similar to the one ofFIG. 5, but with additional through-going openings outside the annularconducting path,

FIG. 7 illustrates yet another SMT embodiment, where a generally annularconducting path is arranged within one through-going opening surroundingthe annular conducting path, but where the annular conductor and theunderlying circuit board part has a cut so as to better allow forthermal movement during manufacturing,

FIGS. 8a and 8b illustrate an SMT embodiment, where an annularconducting path connects to first transducer terminal, while anotherconductor on the circuit board connects to a metal clip that serves toconnect to a second transducer terminal, and

FIGS. 9a and 9b illustrate an SMT embodiment with conducting padsarranged for soldering to respective concentrically positionedelectrical terminals of the ultrasound transducer, thus allowing thetransducer to be fully SMT mounted to the circuit board.

DESCRIPTION OF EMBODIMENTS

FIG. 1a shows a mechanically separate unit 100 comprising a plane singleor multi-layer circuit board 102 with an electronic circuit 104, such asimplemented with SMT mounted components, and with cylindrically shapedpiezoelectric ultrasound transducers 106 mounted spaced apart andoutside a plane defined by the circuit board 102, namely on a surface ofthe circuit board 102. The ultrasound transducers 106 in the shownembodiment are mounted on top of respective through-going holes of thecircuit board 102, however, other embodiments could have cavities whichare not through-going, or have planar surfaces without holes. Theultrasound transducers 106 are arranged for radiating ultrasonic signalsperpendicular to a plane defined by the circuit board 102.

The electronic circuit 104 is arranged to control the operation of theultrasound transducers 106 and thus perform ultrasound flowmeasurements. The electronic circuit 104 may provide an electric outputsignal in the form of electric pulses with a rate corresponding to themeasured flow rate, and/or the electronic circuit 104 may be arranged tooutput data representing a measured amount of flow, such as by means ofa digital electric signal, e.g. on a single wire.

Metallic clips 110 serve as fixing members for fixing a position of theultrasound transducers 106 and for providing electrical connection tothe electronic circuit 104 on the circuit board 102. The metal clips 110may be SMT soldered to conductive paths on the circuit board 102,however the clips 110 may also be attached to the circuit board by othermeans such as conductive glue, by means of a screw through the circuitboard 102 or the like. Depending on the location of the electricalterminals on the ultrasound transducer 106, the two clips 110 shown foreach transducer 106 may either only serve to electrically connect oneelectrical terminal to the circuit board, or the two clips 110 may serveto electrically connect respective first and second electrical terminalsof the ultrasound transducer 106.

The unit 100 including circuit board 102 and transducers 106 can beelectrically connected and tested in a test-stand before being mountedon a housing with a measuring tube inside and thus before final assemblyof the flow meter.

The unit 100 can be used for mounting on flow meter housings with orwithout through-going openings to the fluid medium to be measured on.However, in preferred embodiments, the 100 according to the invention iscombined with a housing without such through-going opening, i.e. basedon transmission of ultrasound signals through the material of thehousing according to the so-called matching layer principle. E.g. thehousing may be a polymeric housing.

FIG. 1b shows a perspective view of a circuit board 102 as in FIG. 1a,but prior to mounting the ultrasound transducers 106 in relation to thecircuit board 102. The clips 110 may be SMT mounted on the circuit board102 in the same process as the electronic components of the electroniccircuit 504, and subsequently the ultrasound transducers 106 are lockedinto place and electrically connected by means of the clips 110.

As seen, the ultrasound transducers 106 are mounted over respectivethrough-going holes 108 of the circuit board, hereby allowing a contactforce to be applied on each of the ultrasound transducers 106, throughthe plane of the circuit board 102, without applying the force throughthe material of the circuit board 102.

FIG. 2a shows two ultrasound transducers 206 and a circuit board 202with an electronic circuit 204 prior to fixing the transducers on thecircuit board in an SMD process. In a preferred embodiment suited forSMT fixing of the transducers 206, the circuit board 502 is providedwith though-going holes 208 with a circular shape having a diametersmaller than the circular transducers 206, and the holes 208 are placedsuch that the transducers 206, when mounted, cover the holes 208.However, other embodiments may have cavities, which are notthrough-going, or plane surfaces without holes or cavities.

FIG. 2b shows a mechanically separate unit 200 comprising a circuitboard 202 with an electronic circuit 204 and two ultrasound transducers206, where the transducers 206 are fixed on the circuit board 202 in anSMT process. Solder between the circuit board 202 and each of thetransducers 206 serves to mechanically fix the transducers 206 to thecircuit board 202. Furthermore, the electrically conducting solderconnects each of the transducers 206 electrically to the electroniccircuit 204.

FIG. 3 shows a perspective view of an ultrasound flow meter with ametallic or polymeric flow meter housing 320, with a measuring tube 314adapted for fluid flow inside, and a mechanically separate partcomprising a circuit board 302 with an electronic circuit 304 and twopiezoelectric ultrasound transducers 306. The mechanically separate unitcomprising the transducers 306 and the circuit board 302 is fixed to thehousing 320 with bracings 316 in order to ensure that enough force isapplied to the transducers in the direction towards a centre line of themeasuring tube in the housing in order to counteract the force appliedin the opposite direction from the pressure of the fluid inside thehousing during operation. Each end of the flow meter housing 320 isprovided with threading 318 in order to facilitate mounting ontoadjacent pipelines. The electronic circuit 304 is provided with externalconnections in the form of pins enabling the electrical connection toother electronic circuits, such as calculator units, power supplies andtelecommunication units. Alternatively, a wireless connection could beused.

FIG. 4 shows a perspective view of an ultrasound flow meter with apolymeric flow meter housing 420, with a through-going opening 414adapted for fluid flow and with a measuring tube arranged inside, and amechanically separate part comprising a circuit board with an electroniccircuit 404 and piezoelectric ultrasound transducers. The polymeric flowmeter housing 420 is monolithically formed in a die casting process andis enclosing a cavity suited for containing components, such as theelectronic circuit 404. This cavity is separated from the through-goingopening 414 by a wall, and the ultrasound transducers on the circuitboard transmit ultrasound signals through this wall and thus operatesaccording to the so-called matching layer principle. The mechanicallyseparate unit comprising the transducers and the circuit board isclamped to the housing 420 with bracing 416. Each end of the flow meterhousing is provided with threading 418 in order to facilitate mountingonto adjacent pipelines. The electronic circuit 404 is provided withexternal connections in the form of pins enabling the electricalconnection to other electronic circuits, such as calculator units, powersupplies and telecommunication units. Alternatively, a wirelessconnection could be used. The shown embodiment can be used as aconsumption meter, such as a water meter, a heat meter or a coolingmeter.

FIGS. 5a and 5b, shows a circuit board 502 embodiment in the form of aunit 500 which can be tested separately, in a pre-assembly state and inan assembled state, respectively. In FIG. 5a a magnified view of thecircuit board 502 is included to better show the conductive path 531arranged around the through-going opening 530 below the ultrasoundtransducers 506. The shown embodiment is suited for SMT mounting, suchas SMT mounting of all electronic components of the measurement circuit504 and the ultrasound transducers 506 in one SMT mounting process whereall components are soldered directly to the circuit board 502. After SMTmounting, the unit 500 is prepared for functional testing, since it canbe connected in a test setup via the electrical terminals 550, i.e. thesame terminals which are used for electrical connection in the finalflow meter or consumption meter, such as connection to a calculatorunit. Alternatively, electrically conductive glue may be used to fix andconnect the ultrasound transducers 506 to the conductive paths 531 ofthe circuit board 502.

As seen, the circuit board 502 is plane in its entire extension, and inprinciple the circuit board 502 may be a simple standard type one, twoor more layer circuit board. The holes 530, and the conductive paths 531around the circular holes 530, serve to determine a position of theultrasound transducers 506, and thus a distance between the twoultrasound transducers 506 in the final flow meter. Therefore, adistance between the holes 530 in the circuit board 502 should beselected to corresponding transducer positions of the flow meterhousing, and also a corresponding distance between ultrasound reflectorsserving to reflect ultrasound between the two ultrasound transducers506. Using SMT mounting of the ultrasound transducers 506, preferably inan automated production line, it is possible to very precisely positionthe two ultrasound transducers 506 in relation to each other, andthereby provide a good fit to the flow meter housing.

In the shown embodiment on FIGS. 5a, 5b, the ultrasound transducers 506are in the form of cylindrically shaped piezoelectric elements withplane end parts. The two electrical terminals of the ultrasoundtransducers 506 are placed on the opposite end parts, one facing thecircuit board 502, and one facing away from the circuit board 502. Thetwo electrical terminals may either be the entire end parts or may beonly part of the end parts, especially the peripheral part. In the shownexample, the conductive path 531 is suited for soldering to theelectrical terminal on a periphery of the end part of the ultrasoundtransducer 506 facing the circuit board 502. The other electricalterminal on the end part of the ultrasound transducer 506 facing awayfrom the circuit board 502 may be connected to the measurement circuit504 by other means, e.g. using wires soldered to the electricalterminals. Alternatively, an electrically conductive membrane coveringboth transducers may be used to establish electrical connection betweenthe electrical terminals facing away from the circuit board 502 of bothtransducers 506, and wherein this electrically conductive membrane isconnected to the electronic circuit 504 e.g. by means of a wire solderedto the membrane.

FIGS. 6a and 6b shows a variant of the circuit board of FIGS. 5a and 5b.A testable unit 600 comprising ultrasound transducers 606 in the form ofcylindrically shaped piezoelectric element, and a circuit board 602 withmeasurement electronics 604 serving to operate the ultrasoundtransducers 606 is provided, preferably by means of an automated SMTprocess, where all electronic components and the ultrasound transducers606 are directly soldered to conductive paths of the circuit board 602.

In the magnified view, the circuit board 602 area where the ultrasoundtransducer 606 is mounted has a circular conductive path 631 suited forconnection to one electrical terminal of the ultrasound transducer 606.A through-going opening in the form of a circular hole 603 is in thisembodiment accompanied by four narrow through-going openings 640 outsidethe area with the circular conductive path 631. These openings 640 serveto facilitate adaptation to extensions and contractions of thepiezo-electric elements 606. As seen, the piezo-electric elements 606cover the holes 630, when mounted, whereas the openings 640 arepositioned outside the area covered by the piezo-electric elements 606.

FIG. 7 illustrates a variant of the circuit board of FIGS. 6a and 6b. InFIG. 7, a generally annular conductor 731 serves for connection to oneterminal of the piezo-electric element. A through-going hole 730 insidethe conductor 731 and a through-going groove 740 outside the conductor731, together with a cut 732 in the conductor 731 and the underlyingpart of the circuit board 702, these holes and grooves serve toeffectively absorb extensions and contractions of the piezo-electricelements 706.

FIGS. 8a and 8b illustrate another circuit board 802 variant, where bothelectrical terminals of the piezo-electric elements 806 are connected tothe circuit board 802, again providing a complete flow measurement unit800 prepared for testing. An annular conductor 831 on a surface of thecircuit board is arranged for soldering to a first electrical terminalof the piezo-electric element 806. This conductor 831 has in itsvicinity a central through-going hole 830, and around its periphery athrough-going groove 840, with the same effect as mentioned for theembodiment of FIG. 7. However, in this embodiment, another conductor 832on the surface of the circuit board 802 is arranged for soldering to ametal clip 810, i.e. a piece of metal bend into shape. This conductor832 is positioned outside a periphery of the circular piezo-electricelement 806 when placed in its intended position on the circuit board.The metal clip 810 can then be soldered to the conductor 832 in the sameSMT process as fixing and electrically connecting the piezo-electricelement 806 to the conductor 831. However, the metal clip 810 mayalternatively be soldered or conductively glued to the conductor 832manually after SMT mounting of the other electrical components.

As seen, the metal clip 810 is bent into shape so as to provideelectrical contact to a peripheral part of the side of thepiezo-electric element 806 facing away from the circuit board, i.e. tothe second electrical terminal of the piezoelectric element 806. Anadvantage of this type of electrical connection of the electroniccircuit to the piezo-electric element 806 is that it is suited for beingcarried out in one single automated SMT mounting process, since the bentmetal clip 810 is not mechanically supported by the piezo-electricelement 806 during the SMT process, and thus the piezo-electric elementand thus the metal clip 810 is not prevented from proper mechanicalcontact with the solder and the electric path on the circuit board 802during the SMT process, which may otherwise lead to a poor electricalconnection between the metal clip 810 and the electronic circuit on thecircuit board 802. In this case electrical contact will be establishedwhen the unit 800 is mounted on a housing, since here the metal clip 810will be pressed against a surface of the housing, thus causing the metalclip 801 to touch the electrical terminal of the piezo-electric element806. However, it is to be understood that the metal clip 810 may also beshaped to provide immediate contact to the piezo-electric element 806,when soldered or fixed to the conductor 832, without any external force,if preferred.

The combination of directly soldering one electrical terminal of thepiezo-electric element 806 to the circuit board 802 and connecting asecond electrical terminal via a metal clip soldered to the circuitboard 802 is advantageous, since full electrical connection of thepiezo-electric element 806 can be established in one single SMT mountingprocess, if preferred.

FIGS. 9a and 9b illustrate a circuit board 902 variant suited forultrasound transducers 906 in the form of circular piezoelectricelements 906 with both electrical terminals 907, 909 placed on a surfaceof one plane side, and thus suited for direct soldering to respectiveconductors 931, 933 on a surface of the circuit board 902. In FIG. 9athe circuit board 902 is seen from the component side, while FIG. 9billustrates the opposite side of the circuit board 902 where theexternal electrical terminal 950 of the flow measurement unit isvisible, here illustrated as a 6-pin plug, however external interfaceswith fewer or more electrical terminals may be preferred. In embodimentswith an on-board battery, such external interface may be purelywireless.

In the illustrated variant, the two electrical terminals 907, 909 areconcentrically arranged on one surface of the piezoelectric element 906.The first electrical terminal 907 has an annular shape and is arrangedon a peripheral part of the plane surface of the piezo-electric element906, while the second electrical terminal 909 is positioned in a centralpart of the surface, inside the first terminals 907. The conductor 933is placed within the conductor 931 and separated by a through-goingopening 930. Outside the conductor 931, a through-going opening 940 isalso provided, with the same effect as previously mentioned.

The magnified view illustrates the corresponding first conductor 931 onthe surface board has an annular shape suited to fit the shape of thefirst terminal 907, while the second conductor 933 is positioned insidethe annular first conductor 931 so as to fit the position of the secondterminal 909.

The embodiment illustrated in FIGS. 9a and 9b is advantageous since itallows full electrical connection of the ultrasound transducer 906 tothe circuit board 902 in one single SMT mounting process withoutinvolving further components than solder. However, the illustratedtransducer type 906 requires the correct side of the transducer 906 toface the circuit board 902 in order to work. However, if this is takeninto account, the mechanical fixing and electrical connection of thetransducer 906 can be combined in an elegant way in one single SMTprocess.

It is to be understood that the details of the arrangement of theelectrically conductive portions on the circuit boards illustrated inFIGS. 5-9 may be combined in any way to provide a desired configuration,depending on the selected type of ultrasound transducer.

The invention provides a highly versatile stand-alone unit for a largevariety of applications where a flow rate of a fluid is to be measured.A simple electrical interface can be provided, e.g. where the measuredflow rate is output as pulses. This interface may be reduced down to onesingle electrical pin, see e.g. European patent application publicationNo. EP 2 042 837, e.g. with part of the casing being a metal serving aselectrical ground connector. Hereby, the unit is easy to fit intodifferent applications. In some embodiments, a battery is providedwithin the water-tight casing, thus allowing a completely self-containedflow meter unit with a simple electric interface.

Further, the range of applications is extended due to the rather compactsize of the unit, especially in versions where the electronic circuit isrefined to allow a high measurement accuracy with a limited distancebetween the first and second ultrasound transducers, e.g. as describedin European patent application EP 2 339 301. This means that the unitcan be used in applications where only a limited space is available forthe flow rate sensor.

Examples of relevant applications are: consumption meters for chargingpurposes, measurement of blood flow in medical examination orsurveillance, ingredient flow within the food industry, flow of water,air or fuel in a machinery (e.g. a vehicle), flow provided to singlesources or groups of heating/cooling elements (or the relatedheating/cooling power based on measurement of the flow) as feedback toheating/cooling control systems.

In all cases, such applications can profit from high precision fluidflow measurements which can be provided at low cost since the flow meteris suited for full or at least highly automated mass production.

To sum up the invention provides an ultrasound flow meter unit arrangedto measure a fluid flow rate with one or more ultrasound transducers(606), and a circuit board (602) with an electronic circuit arranged tooperate the ultrasound transducer(s) (106, 306). The ultrasoundtransducer (606), e.g. in the form of a piezo-electric element, ismechanically fixed to the circuit board (602) by a first electricallyconducting fixing means which additionally serve(s) to provide anelectrical connection between an electrical terminal of the transducer(606) and the electronic circuit. Hereby a functional flow measurementunit (600) is provided which can be tested prior to assembly with a flowmeter housing etc. Preferably, a set of ultrasound transducers (106,306) are soldered directly onto electrically conducting paths (631) on asurface of the circuit board (602) being in electrical connection withthe electronic circuit, e.g. in a single SMT mounting process togetherwith mounting of all other electronic components on the circuit board(602). Through-going openings (630, 640) of the circuit board (602) inthe vicinity of these conducting paths (631) serve to provide thermalelasticity. A metal clip soldered to the circuit board (602) may serveto electrically connect a second electrical terminal of the transducer(606) to the electronic circuit.

In the following, embodiments E1-E18 describe additional possibleembodiments of the invention.

E1. An ultrasound flow meter arranged to measure a flow rate of a fluidflowing through a measuring tube, the flow meter comprising

-   -   a housing (620) with the measuring tube (614) arranged inside,        and    -   a mechanically separate unit (100, 300) arranged in relation to        the housing, the mechanically separate unit (100, 300)        comprising        -   at least one ultrasound transducer (106, 306), and        -   a circuit board (102, 302) with an electronic circuit (104,            304) arranged for operating the at least one ultrasound            transducer (106, 306), wherein at least one fixing member            (110, 310) is attached to the circuit board (102, 302),            wherein the fixing member (110, 310) serves to limit            relative movement between the circuit board (102, 302) and            the ultrasound transducer (106, 306), such as the fixing            member (110, 310) serving to fix the ultrasound transducer            (106, 306) to the circuit board (110, 310).

E2. Ultrasound flow meter according to E1, wherein the ultrasoundtransducer (106, 306) is kept in position by the fixing member (110,310) outside a plane defined by the circuit board (102, 302, such as ona surface of the circuit board (102, 302).

E3. Ultrasound flow meter according to E1, wherein the circuit board(102, 302) comprises an edge (108, 308) shaped to fit at least a part ofthe ultrasound transducer (106, 306), and wherein the ultrasoundtransducer (106, 306) is kept in position by the fixing member (110,310) such that a plane defined by the circuit board (102, 302)intersects through the ultrasound transducer (106, 306), when mountedfor normal operation.

E4. Ultrasound flow meter according to E3, wherein said edge (108, 308)of the circuit board serves, together with the fixing member (110, 310),to fix the position of the ultrasound transducer (106, 306) in relationto the circuit board (102, 302).

E5. Ultrasound flow meter according to E3 or E4, wherein said edge is anedge of a through-going hole (108, 308) through the circuit board (102,302), wherein the through-going hole (108, 308) is shaped to fit a sizeof the ultrasound transducer (106, 306).

E6. Ultrasound flow meter according to any of E1-E5, wherein the fixingmember (110, 310) comprises an electrically conductive portion, andwherein this electrically conductive portion serves to electricallyconnect the circuit board (102, 302) and the ultrasound transducer (106,306).

E7. Ultrasound flow meter according to E6, wherein separate first andsecond parts of the fixing member (110, 310) serve to provide respectivefirst and second electrical connections between the ultrasoundtransducer (106, 306) and the circuit board (102, 302).

E8. Ultrasound flow meter according to any of E1-E7, wherein the fixingmember (110, 310) comprises one or more clips attached to the circuitboard (102, 302), such as one or more metal clips.

E9. Ultrasound flow meter according to any of E1-E8, wherein the fixingmember (110, 310) comprises one or more solderings serving to attach theultrasound transducer (106, 306) to the circuit board (102, 302).

E10. Ultrasound flow meter according to any of E1-E9, comprising firstand second ultrasound transducers (106, 306) fixed to the circuit board(102, 302) by respective first and second parts of the fixing member(110, 310).

E11. Ultrasound flow meter according to E10, wherein the circuit board(302) and the fixing member (310) serve to fix the first and secondultrasound transducers (306) in position relative to each other, andwherein a distance (722) between the first and second ultrasoundtransducers (306) matches a distance (724) between corresponding firstand second transducer receiving positions of the housing (620).

E12. Ultrasound flow meter according to any of E1-E11, wherein thehousing (620, 820) is formed by metal, such as brass or stainless steel,or by a polymeric material.

E13. Consumption meter comprising a flow meter according to any ofE1-E12, wherein the consumption meter is one of: a heating meter, acooling meter, a water meter, or a gas meter.

E14. A method of manufacturing an ultrasound flow meter, the methodcomprising

-   -   forming a mechanically separate unit comprising an ultrasound        transducer and a circuit board with an electronic circuit used        for controlling the ultrasound transducer by mounting the        ultrasound transducer relative to the circuit board by means of        a fixing member serving to limit relative movement between the        ultrasound transducer and the circuit board, and    -   mounting the mechanically separate unit to a housing arranged        for fluid flow.

E15. Method according to claim E14, comprising mounting the ultrasoundtransducer to the circuit board by means of one or more clips, such asmetal clips.

E16. Method according to E14 or E15, comprising soldering the ultrasoundtransducer to the circuit board, such as by means of a Surface Mountingsoldering process.

E17. Method according to any of E14-E16, comprising a step of testing afunction of the mechanically separate unit prior to mounting themechanically separate unit to the housing arranged for fluid flow.

E18. Method according to any of E14-E17, comprising mounting first andsecond ultrasound transducers relative to the circuit board by means ofrespective first and second parts of the fixing member.

Although the present invention has been described in connection withpreferred embodiments, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims.

In this section, certain specific details of the disclosed embodimentsare set forth for purposes of explanation rather than limitation, so asto provide a clear and thorough understanding of the present invention.However, it should be understood readily by those skilled in this art,that the present invention may be practised in other embodiments whichdo not conform exactly to the details set forth herein, withoutdeparting significantly from the spirit and scope of this disclosure.Further, in this context, and for the purposes of brevity and clarity,detailed descriptions of well-known apparatus, circuits and methodologyhave been omitted so as to avoid unnecessary detail and possibleconfusion.

In the claims, the term “comprising” does not exclude the presence ofother elements or steps. Additionally, although individual features maybe included in different claims, these may possibly be advantageouslycombined, and the inclusion in different claims does not imply that acombination of features is not feasible and/or advantageous. Inaddition, singular references do not exclude a plurality. Thus,references to “a”, “an”, “first”, “second” etc. do not preclude aplurality. Reference signs are included in the claims however theinclusion of the reference signs is only for clarity reasons and shouldnot be construed as limiting the scope of the claims.

The invention claimed is:
 1. An ultrasound flow meter unit arranged tomeasure a fluid flow rate, the unit comprising: a first ultrasoundtransducer having a first and a second electrical terminal, and acircuit board with an electronic circuit arranged thereon for operatingthe first ultrasound transducer, wherein the circuit board comprises afirst conducting path electrically connected to the first electricalterminal, and to the electronic circuit, wherein the first ultrasoundtransducer is includes a monolithic body of piezo-electric material withand has one surface in contact with a surface of the circuit board, andwherein the monolithic body is mechanically fixed to the circuit boardby a first electrically conducting fixing means, which provides theelectrical connection between the first electrical terminal and thefirst conducting path wherein the first ultrasound transducer is mountedon top of a through-going hole of the circuit board and surface mountedto the circuit board by fixing the first conducting path arranged atleast partially around a periphery of the through-going hole of thecircuit board to the first electrical terminal arranged on a peripheryof the one surface of the first ultrasound transducer in contact withthe circuit board.
 2. The ultrasound flow meter unit according to claim1, wherein the first fixing means comprises the first conducting path isfixed to the first electrical terminal by using at least one of: solder,or electrically conducting glue, or an electrically conducting clip. 3.The ultrasound flow meter unit according to claim 1, wherein the circuitboard has one or more through-going openings hole is partly or entirelycovered by the first ultrasound transducer when fixed to the circuitboard.
 4. The ultrasound flow meter unit according to claim 3, wherein apart of the first conducting path is arranged in the vicinity of atleast one of the through-going opening(s) in the circuit board.
 5. Theultrasound flow meter unit according to claim 1, wherein the ultrasoundtransducer is mounted on top of a through-going hole of the circuitboard or on top of a non through-going cavity of the circuit board. 6.The ultrasound flow meter unit according to claim 4 1, wherein themonolithic body first ultrasound transducer is fixed to the circuitboard by soldering the first conducting path to the first electricalterminal on a periphery of the end part of the ultrasound transducerfacing the circuit board.
 7. The ultrasound flow meter unit according toclaim 5 1, wherein a the through-going opening hole is accompanied byopenings positioned outside the area covered by the first ultrasoundtransducer when fixed to the circuit board.
 8. The ultrasound flow meterunit according to claim 7, wherein a the through-going opening in theform of hole is a circular hole surrounded by a circular conductingpath, and is accompanied by four narrow through-going openings outsidethe area with the circular conductive path.
 9. The ultrasound flow meterunit according to claim 7, wherein the first conducting path includes agenerally annular conductor with a cut serves for connection to thefirst terminal of the first ultrasound transducer, and wherein a thethrough-going opening hole is arranged inside the conductor and athrough-going grove groove is arranged outside the conductor.
 10. Theultrasound flow meter unit according to claim 7, wherein the firstconducting path includes an annular conductor serves for connection tothe first terminal of the first ultrasound transducer, and wherein a thethrough-going opening hole is arranged inside the conductor and athrough-going grove groove is arranged outside the conductor.
 11. Theultrasound flow meter unit according to claim 1, wherein a secondconducting path on the surface of the circuit board serves toelectrically connect the electronic circuit with to the secondelectrical terminal of the first ultrasound transducer.
 12. Theultrasound flow meter unit according to claim 11, wherein a metal clipserves to electrically connect the second conducting path to the secondelectrical terminal of the first ultrasound transducer.
 13. Theultrasound flow meter unit according to claim 11, wherein the secondconducting path is placed within the first conducting path and separatedby a through-going opening.
 14. The ultrasound flow meter unit accordingto claim 1, wherein the ultrasound flow meter further comprises a secondultrasound transducer, wherein the electronic circuit is furtherarranged for operating the second ultrasound transducer, wherein thesecond ultrasound transducer is mechanically fixed to the circuit boardby a second electrically conducting fixing means, which additionallyserve(s) to provide an electrical connection between at least oneelectrical terminal of the second ultrasound transducer and theelectronic circuit, and wherein both of the first and second ultrasoundtransducers are mechanically fixed to the same side of the circuitboard.
 15. The ultrasound flow meter unit according to claim 14,comprising a substantially watertight membrane arranged around thecircuit board and the first and second ultrasound transducers, andwherein a plug with a plurality of electrical terminals penetratesthrough the membrane so as to allow external connection to theelectronic circuit, wherein at least a part of the membrane iselectrically conducting and serves to electrically connect the secondelectrical terminal of the first ultrasound transducer with a secondelectrical terminal of the second ultrasound transducer.
 16. A flowmeter arranged to measure a flow rate of a fluid flowing through ameasuring tube, wherein the flow meter comprises: a housing with themeasuring tube arranged inside, and an ultrasound flow meter unitaccording to claim 1 arranged in relation to the housing.
 17. The flowmeter according to claim 16, wherein said flow meter is a heat meter, acooling meter, a water meter, or a gas meter.
 18. An ultrasound flowmeter unit arranged to measure a fluid flow rate, the unit comprising: afirst ultrasound transducer including a monolithic body ofpiezo-electric material and having a first and a second electricalterminal; a circuit board with an electronic circuit arranged thereonfor operating the first ultrasound transducer; wherein the firstultrasound transducer is electrically and mechanically connected to thecircuit board by connecting one of the first and second electricterminal to an at least partially annular conductive path of the circuitboard having outside thereof at least one through-going opening orgroove to effectively absorb extensions and contractions of thepiezo-electric material.
 19. The ultrasound flow meter unit of claim 18,wherein the first ultrasound transducer has a cylindrical shape with afirst end surface with a first periphery and a second end surface with asecond periphery, wherein the first end surface faces the circuit board.20. The ultrasound flow meter unit of claim 19, wherein the at leastpartially annular conductive path of the circuit board is at leastpartially surrounded by a void formed in the circuit board.
 21. Theultrasound flow meter unit of claim 20, wherein the void at leastpartially surrounds the first periphery of the first ultrasoundtransducer.
 22. The ultrasound flow meter unit of claim 18, wherein theultrasound flow meter unit comprises a second ultrasound transducer. 23.The ultrasound flow meter unit of claim 22, wherein the secondultrasound transducer includes an additional monolithic body ofpiezo-electric material and having a third and a fourth electricalterminal; wherein the electronic circuit is arranged for operating thesecond ultrasound transducer; wherein the second ultrasound transduceris electrically connected to the electronic circuit arranged on thecircuit board via the third and fourth electrical terminals, and whereinthe second ultrasound transducer is mechanically connected to theelectronic circuit arranged on the circuit board by a second at leastpartially annular conductive path of the circuit board having outsidethereof at least one second through-going opening or groove toeffectively absorb extensions and contractions of the piezo-electricmaterial of the second ultrasound transducer.
 24. The ultrasound flowmeter unit of claim 23, wherein the second ultrasound transducer has acylindrical shape with a third end surface with a third periphery and afourth end surface with a fourth periphery, the third end surface facingthe circuit board, wherein the second ultrasound transducer ismechanically fixed onto a surface of the second at least partiallyannular conductive path of the circuit board, wherein the second atleast partially annular conductive path of the circuit board is arrangedalong the third periphery of the second ultrasound transducer; andwherein the second at least partially annular conductive path of thecircuit board is at least partially surrounded by a second void formedin the circuit board.
 25. The ultrasound flow meter unit of claim 24,wherein the second void at least partially surrounds the third peripheryof the second ultrasound transducer.
 26. A flow meter arranged tomeasure a flow rate of a fluid flowing through a measuring tube, whereinthe flow meter comprises: a housing with the measuring tube arrangedinside, and an ultrasound flow meter unit according to claim 18 arrangedin relation to the housing.
 27. The flow meter according to claim 26,wherein said flow meter is a heat meter, a cooling meter, a water meter,or a gas meter.